1 Field of the Invention
This invention relates generally to offshore drilling systems and more particularly to a pressurized slip joint for use with a marine intervention riser system for workover applications after a well has been drilled. The slip joint enables expeditious operations in the moon pool of a vessel in heavy seas.
2 Background of the Art
Risers for drilling operations typically consist of large diameter pipes extending from the wellhead through an opening in the bottom ("moon pool") of the vessel. Drilling operations are carried out by means of a drill string within the riser. Drilling mud required for drilling is circulated through the drillstring to the drillbit at the bottom of the drillstring, back up the wellbore and through the annulus between the drillstring and the riser. The riser serves to separate the drilling fluid from the surrounding seawater. When drilling operations are carried out in deep water, the danger of buckling of the riser increases. The reason for this is that the riser has the same buckling characteristics as a vertical column and structural failure under compressive loading may occur. To avoid this structural failure, riser tensioning systems are installed on the vessel for applying a tensile force to the upper end of the riser. A variety of such tensioning systems have been used in prior art, including cables, sheaves and pneumatic cylinder mechanisms connected between the vessel and the upper portions of the riser.
Because the riser is fixed at the bottom to the wellhead assembly, wind, wave and tidal action will cause movement of the vessel relative to the top end of the riser. Motion compensating equipment must be incorporated into the tensioning system to maintain the top of the riser within the moon pool. This may include a telescopic coupling arrangement to compensate for heaving motion and a flex joint within the riser to compensate for lateral movement of the vessel. During drilling, pressure inside the riser pipe is comparatively low. However, the pressure may increase if a shallow pocket if gas is encountered and the sliding joint is typically designed to withstand a pressure of 2000 psi or less.
In the case of producing wells, however, the pressure inside the riser can easily approach 10000 psi. Fixed production platforms do not require telescopic risers. In deeper waters, tension leg platforms have been used. Such platforms are subject to more motion than fixed platforms and the risers have to be designed accordingly. On marginal fields where the cost of a production platform would be prohibitive, drilling vessels have been used for production. Production riser pipes for mobile production platforms have been constructed as an integrated unit suspended in tension systems and guides, capable of absorbing the necessary telescopic, lateral and angular movements. U.S. Pat. No. 5,069,488 discloses a telescopic device that is volume and pressure balanced for mobile production platforms. Because of the requirement of no relative vertical motion between the riser and the production vessel, the telescopic system has to be designed to withstand the maximum motion expected in heavy seas.
Marine intervention riser systems are functionally similar to risers used with mobile production platforms in terms of the pressures that are encountered. However, there is one major difference: workover operations typically require a variety of devices to be inserted into the well. Use of these devices requires a considerable amount of human involvement in the vessel. Any system in which the riser pipes in the moon pool have a large vertical movement with respect to the vessel presents a serious safety hazard when humans are preforming workover operations in the vessel. At these times, it is desirable to have no movement between the top of the riser assembly within the moon pool and the vessel. At other times, when humans are not involved, vertical movement of the riser within the moon pool is acceptable: at such times, a system that allows relative motion between the top of the riser assembly within the moon pool and the vessel is acceptable. The present invention is capable of meeting these requirements.